The unique advantages of a stochastic signal method developed in this laboratory will be capitalized on in this study of the semitendinosus and anterior tibialis muscles of the frog: 1) to afford quantitative measurements of their viscoelastic properties and, 2) to increase our understanding of the contractile apparatus in skeletal muscle. The interrogating signal used will be a minute, brief pseudorandom length perturbation which elicits a particular tension response from the muscle. Analysis of these input and output signals directly identifies the dynamic modulus of the preparation over a wide frequency range. Viscoelastic models may then be derived from these moduli without the need to assume any model from beforehand, and the parameters of the elements of the model may be assigned values expressed in physical units. The models and changes in their parameters will be investigated at rest and during isometric tetanic contraction at different mean, central sarcomere lengths measured by laser diffraction in single fibers, small fiber bundles and whole muscles. Analysis of the data will recognize the nonuniform sarcomere spacing along the length of the muscles by measurement of this distribution and by experiments designed to assess the longitudinal uniformity of the perturbing strain. This information will be critically examined to determine what portion of the results found for the entire muscle specimen can be extended to the level of the sarcomere and thereby related to the sliding filament model for contraction. Certain interventions will be used to identify those elements of the model which arise from active or passive sources.